Color filter array with reference pixel to reduce spectral crosstalk

ABSTRACT

A color filter array includes a plurality of tiled minimal repeating units, each minimal repeating unit comprising an M×N set of individual filters. Each minimal repeating unit includes a plurality of imaging filters including individual filters having at least first, second, and third photoresponses, and at least one reference filter having a reference photoresponse, wherein the reference filter is positioned among the imaging filters and wherein the reference photoresponse transmits substantially the crosstalk spectrum that is not filtered from light incident on the color filter array by the plurality of imaging filters. Other embodiments are disclosed and claimed.

TECHNICAL FIELD

The disclosed embodiments relate generally to image sensors and inparticular, but not exclusively, to image sensors including a colorfilter array including a reference pixel to reduce spectral crosstalk.

BACKGROUND

Image sensors can capture color images by combining a pixel array ofphotosensitive pixels with a set of color filters. Each pixel in thepixel array is coupled to a filter that removes all but a particularwavelength—that is, a particular color of light—from the light incidenton the image sensor. As a result, each pixel in the pixel image captureslight of a single color, and for each pixel the color values for theother colors are interpolated.

Spectral crosstalk is a problem that occurs in devices that filterseparate wavelengths, as do color filters that are coupled to a pixelarray in a color image sensor when a small portion of the optical powerthat should have ended up in a particular channel—that is, that shouldhave been output by particular filter—actually ends up in anotherchannel, such as an adjacent channel. In an image sensor with a colorfilter array, spectral crosstalk is often the result of imperfect colorfiltering: a blue filter, for example, might let through some red andgreen light, so that a pixel to which the blue filter is coupled willreceive more than just the blue light it is supposed to receive. In aCMOS image sensor, crosstalk causes degradation of spatial resolution,color mixing, and image noise. This can have serious effects on thesensor's signal-to-noise ratio (SNR) and hence on the error rate of thesystem. In an RGB image sensor, sensitivity came can be gained by usinga clear filter in the filter array, but it increases crosstalk.

Various methods have been tried to decrease spectral crosstalk, but allhave drawbacks. Some other types of crosstalk can be reduced by processprofile control between pixels, but spectral crosstalk is a naturalmaterial property that can't be reduced by process. Another way toreduce spectral crosstalk is to use thicker color filters; this cansignificantly reduce crosstalk, but it also reduces the signal strengthand the quantum efficiency (QE) of the pixel. Another way to reducespectral crosstalk is to increase pigment concentration in the filters,but the material's refractive index will increase and induce higheroptical crosstalk.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a schematic of an embodiment of an image sensor including acolor filter array.

FIGS. 2A-2B are, respectively, cross-sections of embodiments of a pairof frontside illuminated pixels and embodiments of a pair ofbackside-illuminated pixels.

FIGS. 3A-3C are diagrams that explain the terminology used to describeembodiments of color filter arrays and minimal repeating units.

FIGS. 4A-4B are plan view drawings of an embodiment of a minimalrepeating unit (FIG. 4A) and a corresponding color filter array (FIG.4B).

FIG. 5 is a graph illustrating spectral crosstalk in an embodiment of acolor filter array.

FIGS. 6A-6B are graphs illustrating an embodiment of correcting spectralcrosstalk in a color filter array.

FIGS. 7A-7B are plan view drawings of an embodiment of a minimalrepeating unit including a reference pixel (FIG. 7A) and a correspondingcolor filter array (FIG. 7B).

FIGS. 8A-8B are plan view drawings of an embodiment of a minimalrepeating unit including a reference pixel (FIG. 8A) and a correspondingcolor filter array (FIG. 8B).

FIG. 9A is a plan view drawing of an embodiment of a red-green-blue(RGB) minimal repeating unit including a reference pixel and acorresponding color filter array.

FIG. 9B is a plan view drawing of an embodiment of a cyan-magenta-yellow(CMY) minimal repeating unit including a reference pixel and acorresponding color filter array.

FIG. 10 is a flowchart of an embodiment of a process for correctingimages to account for spectral crosstalk.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

Embodiments are described of an apparatus, system and method for imagesensors including a color filter array with a reference pixel to reducespectral crosstalk. Specific details are described to provide a thoroughunderstanding of the embodiments, but one skilled in the relevant artwill recognize that the invention can be practiced without one or moreof the described details, or with other methods, components, materials,etc. In some instances, well-known structures, materials, or operationsare not shown or described in detail but are nonetheless encompassedwithin the scope of the invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a described feature, structure, or characteristiccan be included in at least one described embodiment, so thatappearances of “in one embodiment” or “in an embodiment” do notnecessarily all refer to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

FIG. 1 illustrates an embodiment of a complementary metal oxidesemiconductor (CMOS) image sensor 100 including a color pixel array 105,readout circuitry 170 coupled to the pixel array, function logic 115coupled to the readout circuitry, and control circuitry 120 coupled tothe pixel array. Color pixel array 105 can be implemented in afrontside-illuminated image sensor, as shown in FIG. 2A, or as abackside-illuminated image sensor, as shown in FIG. 2B. Color pixelarray 105 is a two-dimensional (2D) array of individual imaging sensorsor pixels (e.g., pixels P1, P2 . . . , Pn) arranged into X pixel columnsand Y pixel rows. As illustrated, each individual pixel in the array isarranged into a row (e.g., rows R1 to Ry) and a column (e.g., column C1to Cx) to acquire image data of a person, place, or object. The imagedata or pixel data can then be used to render a 2D image of the person,place, or object. Color pixel array 105, if present, assigns color toeach pixel using a color filter array (“CFA”) coupled to the pixelarray.

After each pixel in pixel array 105 has acquired its image data or imagecharge, the image data is read out from the individual pixels by readoutcircuitry 170 and transferred to function logic 115 for storage,additional processing, etc. Readout circuitry 170 can includeamplification circuitry, analog-to-digital (“ADC”) conversion circuitry,or other circuits. Function logic 115 can store the image data and/ormanipulate the image data by applying post-image effects (e.g., crop,rotate, remove red eye, adjust brightness, adjust contrast, high dynamicrange (HDR) image combination, or otherwise). Function logic 115 canalso be used in one embodiment to process the image data to correct(i.e., reduce or remove) fixed pattern noise. Control circuitry 120 iscoupled to pixel array 105 to control operational characteristic of thepixel array 105. For example, control circuitry 120 can generate ashutter signal for controlling image acquisition.

FIG. 2A illustrates a cross-section of an embodiment of a pair offrontside-illuminated (FSI) pixels 200 in a CMOS image sensor. The frontside of FSI pixels 200 is the side of substrate 202 upon which thephotosensitive area 204 and associated pixel circuitry are disposed, andover which metal stack 206 for redistributing signals is formed. Metalstack 206 includes metal layers M1 and M2, which are patterned to createan optical passage through which light incident on FSI pixels 200 canreach photosensitive or photodiode (“PD”) regions 204. To implement acolor image sensor, the front side can include color filter arrangement201, with each of its individual color filters (individual filters 203and 205 are illustrated in this particular cross section) disposed undera microlens 206 that aids in focusing incident light onto PD region 204.

FIG. 2B illustrates a cross-section of an embodiment of a pair ofbackside-illuminated (BSI) pixels 250 in a CMOS image sensor. As in FSIpixels 200, the front side of pixels 250 is the side of substrate 202upon which the photosensitive regions 204 and associated pixel circuitryare disposed, and over which metal stack 206 is formed forredistributing signals. The backside is the side of substrate 202opposite the front side. To implement a color image sensor, the backsidecan include color filter array 201, with each of its individual colorfilters (individual filters 203 and 205 are illustrated in thisparticular cross section) disposed under a microlens 206. Microlenses206 aid in focusing incident light onto photosensitive regions 204.Backside illumination of pixels 250 means that the metal interconnectlines in metal stack 206 do not obscure the path between the objectbeing imaged and the photosensitive regions 204, resulting in greatersignal generation by photosensitive regions 204.

FIGS. 3A-3C illustrate terminology that will be used in the discussionof color filter arrays (CFAs) and minimal repeating units (MRUs). FIG.3A illustrates an embodiment of a CFA 300. CFA 300 includes a number ofindividual filters. Each individual filter has a particularphotoresponse and is optically coupled to a corresponding individualpixel in the pixel array. As a result, each pixel has a particular colorphotoresponse selected from a set of photoresponses. A particularphotoresponse has high sensitivity to certain portions of theelectromagnetic spectrum while simultaneously having low sensitivity toother portions of the spectrum. Because CFAs assign a separatephotoresponse to each pixel by placing a filter over the pixel, it iscommon to refer to a pixel as a pixel of that particular photoresponse.Hence a pixel can be referred to as a “clear pixel” if it has no filteror is coupled to a clear (i.e., panchromatic or colorless) filter, as a“blue pixel” if it is coupled to a blue filter, as a “green pixel” if itis coupled to a green filter, or as a “red pixel” if it is coupled to ared filter, and so on.

The set of color photoresponses selected for use in a sensor usually hasat least three colors but can include four or more in some embodiments.In one embodiment the set of color photoresponses can be red, green,blue, and clear or panchromatic (i.e., neutral or colorless). In otherembodiments, CFA 300 can include other photoresponses in addition to, orinstead of, those listed. For instance, other embodiments can includecyan (C), magenta (M), and yellow (Y) filters, clear (i.e., colorless)filters, infrared filters, ultraviolet filters, x-ray filters, etc.Other embodiments can also include a filter array with an MRU thatincludes a greater or lesser number of pixels than illustrated for MRU302. As used herein, a white, clear, or panchromatic photoresponserefers to a photoresponse having a wider spectral sensitivity than thosespectral sensitivities represented in the selected set of colorphotoresponses. A panchromatic photosensitivity can have highsensitivity across the entire visible spectrum. The term panchromaticpixel refers to a pixel with a panchromatic photoresponse. Although thepanchromatic pixels generally have a wider spectral sensitivity than theset of color photoresponses, each panchromatic pixel can have anassociated filter. Such filter can be either a neutral density filter ora color filter

The individual filters in CFA 300 are grouped into minimal repeatingunits (MRUs) such as MRU 302, and the MRUs are tiled vertically andhorizontally, as indicated by the arrows, to form CFA 300. A minimalrepeating unit is a repeating unit such that no other repeating unit hasfewer individual filters. A given color filter array can include severaldifferent repeating units, but a repeating unit is not a minimalrepeating unit if there is another repeating unit in the array thatincludes fewer individual filters.

FIG. 3B illustrates an embodiment of an MRU 302. MRU 302 is an array ofindividual filters grouped into rows and columns. MRU 302 includes Mcolumns and N rows, with columns measured by index i and rows measuredby index j, so that i ranges from 1 to M and j ranges from 1 to N. Inthe illustrated embodiment MRU 302 is square, meaning that N=M, but inother embodiments M and N need not be equal. MRU 302 can be divided intofour quadrants, with first through fourth quadrants numbered I-IV andarranged counterclockwise starting from the top right: quadrant I in theupper right, quadrant II in the upper left, quadrant III in the lowerleft, and quadrant IV in the lower right.

FIG. 3C illustrates terminology used below to describe the disclosedMRUs, but the terminology can also be used to describe the color filterarray (CFA) as a whole. In a 4×4 MRU (M=N=4)A major diagonal runs fromupper left to lower right, whereas a minor diagonal runs from upperright to lower left. The four pixel long diagonal that runs from topleft to bottom right is known as the major long diagonal. Above themajor long diagonal, the two pixel diagonal that runs from upper left tolower right is known as the upper major short diagonal. Below the majorlong diagonal, the two pixel diagonal that runs from upper left to lowerright is known as the lower major short diagonal. The terminology usedfor minor diagonals would be similar, as shown in the figure.

FIGS. 4A-4B illustrate an embodiment of a minimal repeating unit (MRU)402 and its corresponding color filter array (CFA) 404. Using theterminology of FIG. 3B-3C, FIG. 4A illustrates a sixteen-filter MRU thatis 4×4, meaning that it is square with M=N=4. In MRU 402 quadrants I andIII have clear filters along their minor diagonals and green filtersalong their major diagonals. Quadrant II has clear filters along itsminor diagonal and red filters along its major diagonal. Finally,quadrant IV has clear filters along its minor diagonal and blue filtersalong its major diagonal. FIG. 4B illustrates an embodiment of a CFA 404formed by tiling MRU 402 as shown in FIG. 3A.

FIG. 5 illustrates the problem of spectral crosstalk in an embodiment ofa red-green-blue-clear (RGBC) color filter array such as CFA 404. In thefigure the horizontal axis represents the wavelength of incident light(i.e., its color) while the vertical axis represents the percentagetransmission of that particular wavelength. The graph, then, shows whatpercentage of a particular wavelength of light is transmitted by filterof a particular color.

As an example, when the incident light is at a wavelength of 500-550 nm,which substantially corresponds to green light, the green filter passesnearly 65% of the incident light. Ideally the blue and red filter shouldpass none of this incident green light because they should filter outall wavelengths that are not blue or red. But as can be seen in thecircled portion labeled “spectral crosstalk,” at 500-550 nm the blue andred filters nonetheless transmit about 5% of the incident green light.The same is true of the other color filters: looking at the blue filterat an incident wavelength around 400-450 nm, which substantiallycorresponds to blue light, the blue filter passes about 65% of theincident light at that wavelength. But as can be seen in the circledportion labeled “spectral crosstalk,” at 400-450 nm the green and redfilters pass about 5% of the incident blue light. This imperfectfiltering is a cause of spectral crosstalk. A similar effect happenswith green and blue filters at incident wavelengths around 600-650 nm,which substantially corresponds to red light.

FIGS. 6A-6B are graphs illustrating an embodiment of a process forreducing or eliminating spectral crosstalk. As in FIG. 5, in FIGS. 6A-6Bthe horizontal axis represents the wavelength of incident light (i.e.,its color) while the vertical axis represents the percentagetransmission of that particular wavelength. FIGS. 6A-6B, then, show whatpercentage of light of a particular wavelength is transmitted by filterof a given color. FIG. 6A is a graph illustrating the use of a referencepixel. An embodiment of a solution for optical crosstalk is to use apixel optically coupled to a reference filter (Re) (together known as a“reference pixel”) that transmits the crosstalk spectrum. That is, atany given wavelength of incident light the reference filter Re transmitssubstantially the same or similar percentage of that wavelength that isunfiltered by filters of a different color than the incident wavelength.As an example, if the incident light is in the wavelength range of500-550 nm, the Re filter passes around 5% of the incident light, whichsubstantially corresponds to the percentage of incident green lightpassed by the red and blue filters.

FIG. 6B illustrates an embodiment of how a reference pixel—that is, apixel optically coupled to a reference filter Re as illustrated in FIG.6A—can be used to reduce or eliminate spectral crosstalk. The left graphsubstantially duplicates FIG. 6A, showing the filtering spectrum ofreference filter Re, while the right graph shows that the percentagetransmission of the reference pixel is subtracted from the red, green,and blue pixels so that the red, green, and blue values are shifted downby an amount Δ. Shifting the red, green, and blue values thatsubstantially reduces or eliminates the spectral crosstalk, as can beseen in the graph on the right. In the illustrated embodiment, Δ isshown as a single value by which all the red, green, and blue values areshifted downward. But in other embodiments Δ need not be a single value,but can instead vary by wavelength, such that red, green, and bluevalues can be shifted down by different Δs. Other ways of determining Δscan be used in other embodiments.

FIGS. 7A-7B illustrate an embodiment of a minimal repeating unit (MRU)702 that includes a reference filter (Re), as well as a correspondingcolor filter array (CFA) 704. Again using the terminology of FIGS.3A-3C, FIG. 7A illustrates a 16-filter MRU 702 that is 4×4, meaning thatit is square with M=N=4. MRU 702 is similar to MRU 302: quadrant II hasclear filters along its minor diagonal and red filters along its majordiagonal; quadrant III has clear filters along its minor diagonal andgreen filters along its major diagonal; and quadrant IV has clearfilters along its minor diagonal and blue filters along its majordiagonal. The primary difference between MRUs 302 and 702 is thatquadrant I replaces one of the clear filters with a reference filter Re,so that quadrant I has a clear filter and a reference filter along itsminor diagonal and green filters along its major diagonal. FIG. 7Billustrates an embodiment of a CFA 704 formed by tiling MRU 702 as shownin FIG. 3A.

FIGS. 8A-8B illustrate another embodiment of a minimal repeating unit(MRU) 802 that includes a reference filter (Re), as well as acorresponding color filter array (CFA) 804. FIG. 8A illustrates a64-filter MRU that is 8×8, meaning that MRU 802 is square with M=N=8. InMRU 802, quadrants I-IV have two red and two blue filters along theirmajor diagonal. Quadrants I and III have clear filters on their minordiagonals, except for a reference filter positioned on the minordiagonal along the lower major short diagonal. Quadrants II and IV haveclear filters on their minor diagonal, except for a reference filterpositioned on the minor diagonal along the upper major short diagonal.FIG. 8B illustrates an embodiment of a CFA 804 formed by tiling MRU 802as shown in FIG. 3A.

FIGS. 9A-9B illustrate other embodiments of MRUs, and theircorresponding CFAs, that include reference filters. All the precedingMRU and CFA embodiments are RGBC patterns—they all include red, green,blue, and clear filters—but in other embodiments that need not be thecase. FIG. 9A illustrates another MRU 902 that is an RGB pattern (i.e.,it includes no clear pixel), but nonetheless includes a reference pixel.MRU 902 has four filter and is 2×2, meaning that it is square withM=N=2. MRU 902 has a reference filter and a green filter along its majordiagonal and blue and red filters along its minor diagonal. MRU 902 isthe well-known Bayer filter pattern, but with one green filter replacedwith a reference filter. In the illustrated embodiment, a green filteris replaced with a reference filter because a Bayer pattern normallyincludes two green filters, so that replacing one green still leavesanother in the pattern. But this need not be the case in everyembodiment. In other embodiments, for example, the filter color thatmost closely approximates the luminance component of the image can bereplaced with a reference pixel. The right side of the figureillustrates a CFA 904 formed by tiling MRU 902 as shown in FIG. 3A.

FIG. 9B illustrates another MRU 906 that is a cyan-magenta-yellow (CMY)pattern (i.e., it includes no clear pixel), but nonetheless includes areference pixel. MRU 906 is 2×2, meaning that it is square with M=N=2.MRU 906 has a reference filter and a yellow filter along its majordiagonal and cyan and magenta filters along its minor diagonal. MRU 906is similar to MRU 902, but with a different primary color set: the RGBcolor set of MRU 902 has been replaced with a CMY primary color set, andwith one of the yellow filters replaced by a reference filter. In theillustrated embodiment, a yellow filter is replaced with a referencefilter because there would usually be two yellow filters in the pattern,so replacing one yellow still leaves another in the pattern. But inother embodiments the filter color that most closely approximates theluminance component of the image can be replaced with the referencepixel. The right side of the figure illustrates a CFA 908 formed bytiling MRU 906 as shown in FIG. 3A.

FIG. 10 illustrates an embodiment of a process 1000 for correctingspectral crosstalk in an image. The process starts at block 1002. Atblock 1004 the process captures an image using a pixel array opticallycoupled to a color filter array with a reference filter, such as any ofthe color filter arrays shown in preceding figures. The pixel array canbe thought of as composed of two types of pixels: imaging pixels (e.g.,pixels coupled to R, G, B, C filters) that generate imaging pixel valueson which the final image will be based, and reference pixels thatgenerate reference pixel values. Initially, each pixel in the pixelarray will generate a single pixel value that is either imaging pixelvalue or a reference pixel value depending on what type of pixel it is.

At block 1006, the first imaging pixel is selected for processing. Atblock 1008 the imaging pixel value for the selected pixel is retrieved,and at block 1010 the imaging pixel value is adjusted using the relevantreference pixel value. In one embodiment the imaging pixel value isadjusted by simply subtracting the reference pixel value of thereference pixel closest to the imaging pixel whose value is beingprocessed. The closest reference pixel need not be within the same MRUas the imaging pixel being processed. In another embodiment, acorrection Δ can be computed based on two or more of the closestreference pixels to the imaging pixel whose imaging pixel value is beingadjusted—for example the 2, 3, 4, etc., closest reference pixels. Forexample, in one embodiment Δ could be a weighted average of the nearestreference pixel values. Different embodiments can weight the referencepixel values differently; in one embodiment, for example, the referencepixel values can be weighted based on distance from the imaging pixelbeing corrected and intensity.

At block 1012, the adjusted imaging pixel value is stored, after whichthe process continues to block 1014 and checks to see if more imagingpixels remain to be processed. If at block 1014 more imaging pixelsremain to be processed, the process continues to block 1016 where itselects the next imaging pixel and then returns to block 1008 andrepeats the process for the newly-selected imaging pixel. If at block1014 no imaging pixels remain to be processed, meaning that all imagingpixel values in the image have been adjusted, the process continues toblock 1018. At block 1018, color values are interpolated for all imagingpixels to form a final image, so that in the final image all pixels willhave a set of three imaging pixel values corresponding to a primarycolor set (e.g., red, green, and blue in an RGB or RGBC). After imagingpixel values are interpolated for the imaging pixels, the image isoutput at block 1020 and the process ends at block 1022.

The above description of illustrated embodiments of the invention,including what is described in the abstract, is not intended to beexhaustive or to limit the invention to the exact forms disclosed. Whilespecific embodiments of, and examples for, the invention are describedherein for illustrative purposes, various equivalent modifications arepossible within the scope of the invention, as those skilled in therelevant art will recognize. These modifications can be made to theinvention in light of the above detailed description.

The terms used in the following claims should not be interpreted tolimit the invention to the specific embodiments disclosed in thespecification and the claims. Rather, the scope of the invention is tobe determined entirely by the following claims, which are to beinterpreted in accordance with established doctrines of claiminterpretation.

The invention claimed is:
 1. A color filter array comprising: a plurality of tiled minimal repeating units, each minimal repeating unit comprising an M×N set of individual filters, wherein each minimal repeating unit includes: a plurality of imaging filters including individual filters having at least first, second, third, and fourth photoresponses; and at least one reference filter having a reference photoresponse, wherein the reference filter is positioned among the imaging filters and wherein the reference photoresponse transmits substantially the crosstalk spectrum that is not filtered from light incident on the color filter array by the plurality of imaging filters; wherein the first photoresponse is panchromatic (C), the second photoresponse is red (R), the third photoresponse is green (G), and the fourth photoresponse is blue (B), and wherein the minimal repeating unit is selected from a group consisting of a minimal repeating unit wherein M=N=4 and the minimal repeating unit is: R C G C C R Re G G C B C C G C B

and a minimal repeating unit wherein M=N=8 and the minimal repeating unit is: R C G C R C G C C R Re G C R C G G C B C G Re B C C G C B C G C B R C G C R C G C C R C G C R Re G G Re B C G C B C C G C B C G C B

wherein Re represents a filter with the reference photoresponse.
 2. A color filter array comprising: a plurality of tiled minimal repeating units, each minimal repeating unit comprising an M×N set of individual filters, wherein each minimal repeating unit includes: a plurality of imaging filters including individual filters having at least first, second, and third photoresponses; and at least one reference filter having a reference photoresponse, wherein the reference filter is positioned among the imaging filters and wherein the reference photoresponse transmits substantially the crosstalk spectrum that is not filtered from light incident on the color filter array by the plurality of imaging filters; wherein the first photoresponse is red (R), the second photoresponse is green (G), and the third photoresponse is blue (B), and wherein M=N=2 and the minimal repeating unit is: Re R B G

wherein Re represents a filter with the reference photoresponse.
 3. An image sensor comprising: a pixel array including a plurality of individual pixels; a color filter array positioned over and optically coupled to the pixel array, the color filter array comprising a plurality of tiled minimal repeating units, each minimal repeating unit comprising an M×N set of individual filters, wherein each minimal repeating unit includes: a plurality of imaging filters including individual filters having at least first, second, third, and fourth photoresponses, and at least one reference filter having a reference photoresponse, wherein the reference filter is positioned among the imaging filters and wherein the reference photoresponse transmits substantially only the crosstalk spectrum that is not filtered from light incident on the color filter array by the plurality of imaging filters; wherein the first photoresponse is panchromatic (C), the second photoresponse is red (R), the third photoresponse is green (G), and the fourth photoresponse is blue (B), wherein the minimal repeating unit is selected from a group consisting of a minimal repeating unit wherein M=N=4 and the minimal repeating unit is: R C G C C R Re G G C B C C G C B

and a minimal repeating unit wherein M=N=8 and the minimal repeating unit is: R C G C R C G C C R Re G C R C G G C B C G Re B C C G C B C G C B R C G C R C G C C R C G C R Re G G Re B C G C B C C G C B C G C B

wherein Re represents a filter with the reference photoresponse; and readout circuitry coupled to the pixel array to read out signals from the individual pixels.
 4. An image sensor comprising: a pixel array including a plurality of individual pixels; a color filter array positioned over and optically coupled to the pixel array, the color filter array comprising a plurality of tiled minimal repeating units, each minimal repeating unit comprising an M×N set of individual filters, wherein each minimal repeating unit includes: a plurality of imaging filters including individual filters having at least first, second, and third photoresponses, and at least one reference filter having a reference photoresponse, wherein the reference filter is positioned among the imaging filters and wherein the reference photoresponse transmits substantially only the crosstalk spectrum that is not filtered from light incident on the color filter array by the plurality of imaging filters; wherein the first photoresponse is red (R), the second photoresponse is green (G), and the third photoresponse is blue (B), and wherein M=N=2 and the minimal repeating unit is: Re R B G

wherein Re represents a filter with the reference photoresponse; and readout circuitry coupled to the pixel array to read out signals from the individual pixels.
 5. A process comprising: capturing an image comprising a plurality of imaging pixel values and a plurality of reference pixel values using a pixel array including a plurality of individual pixels and optically coupled to a color filter array positioned over the pixel array, the color filter array comprising a plurality of tiled minimal repeating units, each minimal repeating unit comprising an M×N set of individual filters, wherein each minimal repeating unit includes: a plurality of imaging filters, each imaging filter coupled to an imaging pixel, wherein the plurality of imaging filters includes individual filters having at least first, second, third, and fourth photoresponses, and at least one reference filter optically coupled to a reference pixel, the reference filter having a reference photoresponse, wherein the reference filter is positioned among the imaging filters and wherein the reference photoresponse transmits substantially only the crosstalk spectrum that is not filtered from light incident on the color filter array by the plurality of imaging filters; wherein the first photoresponse is panchromatic (C), the second photoresponse is red (R), the third photoresponse is green (G), and the fourth photoresponse is blue (B), and wherein the minimal repeating unit is selected from a group consisting of a minimal repeating unit wherein M=N=4 and the minimal repeating unit is: R C G C C R Re G G C B C C G C B

and a minimal repeating unit wherein M=N=8 and the minimal repeating unit is: R C G C R C G C C R Re G C R C G G C B C G Re B C C G C B C G C B R C G C R C G C C R C G C R Re G G Re B C G C B C C G C B C G C B

wherein Re represents a filter with the reference photoresponse; and adjusting imaging pixel values based on one or more reference pixel values to reduce or remove spectral crosstalk from the imaging pixel values.
 6. The process of claim 5 wherein the one or more reference pixel values need not be from reference pixels within the same minimal repeating unit.
 7. The process of claim 5 wherein adjusting the values of the imaging pixels to reduce or remove spectral crosstalk comprises, for each imaging pixel, subtracting the reference pixel value of the nearest reference pixel.
 8. A process comprising: capturing an image comprising a plurality of imaging pixel values and a plurality of reference pixel values using a pixel array including a plurality of individual pixels and optically coupled to a color filter array positioned over the pixel array, the color filter array comprising a plurality of tiled minimal repeating units, each minimal repeating unit comprising an M×N set of individual filters, wherein each minimal repeating unit includes: a plurality of imaging filters, each imaging filter coupled to an imaging pixel, wherein the plurality of imaging filters includes individual filters having at least first, second, and third photoresponses, and at least one reference filter optically coupled to a reference pixel, the reference filter having a reference photoresponse, wherein the reference filter is positioned among the imaging filters and wherein the reference photoresponse transmits substantially only the crosstalk spectrum that is not filtered from light incident on the color filter array by the plurality of imaging filters; wherein the first photoresponse is red (R), the second photoresponse is green (G), and the third photoresponse is blue (B) and wherein M=N=2 and the minimal repeating unit is: Re R B G

wherein Re represents a filter with the reference photoresponse; and adjusting imaging pixel values based on one or more reference pixel values to reduce or remove spectral crosstalk from the imaging pixel values.
 9. The process of claim 8 wherein adjusting the values of the imaging pixels to reduce or remove spectral crosstalk comprises, for each imaging pixel, subtracting the reference pixel value of the nearest reference pixel.
 10. The process of claim 8 wherein the one or more reference pixel values need not be from reference pixels within the same minimal repeating unit.
 11. A process comprising: capturing an image comprising a plurality of imaging pixel values and a plurality of reference pixel values using a pixel array including a plurality of individual pixels and optically coupled to a color filter array positioned over the pixel array, the color filter array comprising a plurality of tiled minimal repeating units, each minimal repeating unit comprising an M×N set of individual filters, wherein each minimal repeating unit includes: a plurality of imaging filters, each imaging filter coupled to an imaging pixel, wherein the plurality of imaging filters includes individual filters having at least first, second, and third photoresponses, and at least one reference filter optically coupled to a reference pixel, the reference filter having a reference photoresponse, wherein the reference filter is positioned among the imaging filters and wherein the reference photoresponse transmits substantially only the crosstalk spectrum that is not filtered from light incident on the color filter array by the plurality of imaging filters; adjusting imaging pixel values based on one or more reference pixel values to reduce or remove spectral crosstalk from the imaging pixel values, wherein adjusting the values of the imaging pixels to reduce or remove spectral crosstalk comprises, for each imaging pixel, subtracting from the imaging pixel value a weighted average reference pixel values of two or more of the nearest reference pixels. 